Transcranial magnetic stimulation (“TMS”) uses an induction coil to induce an electric field (“E-field”) within the brain. The locations of the brain exposed to a strong enough E-field will become activated, or stimulated. In navigated brain stimulation (“NBS”), the E-field induced in the brain by a TMS induction coil device is graphically represented on a display. As part of NBS, a three-dimensional (“3D”) localization system is used to locate the TMS coil device accurately with respect to a subject's head. The localization system correlates TMS coil device location information with anatomical information representative of a subject's brain, which typically is obtained from magnetic resonance imaging (“MRI”) of the brain. The E-field information is shown as an overlay on a graphical display of the subject's brain generated from the MRI images of the brain. By viewing the display, the user can interactively position the TMS coil device, in real time, in relation to the brain to stimulate a desired location of the brain.
A TMS induction coil device typically includes coils having 5 to 30 loops (windings) of copper wire located in a casing. The windings are normally circularly shaped or in the form of a FIG. 8. The shape, and the location of the maximum, of the E-field induced in the brain depend on the exact shape of the coil windings within the TMS coil device and their location and orientation with respect to the brain. In NBS, the strength and location of the E-field induced in the brain by the TMS coil device is determined from information representative of the location and orientation of the casing of the TMS coil device in relation to the brain and the location and orientation of the coil windings within and in relation to, respectively, the casing. The location and orientation of the casing is obtained from a navigation or tracking device, such as an infrared tracking device including an infrared transceiver and infrared reflective elements attached to the TMS coil device, that tracks the movement of the casing, as is conventional in the art. The location and orientation of the coil windings within the casing are determined by generating a model of the coil windings within the casing of the TMS coil device using information obtained from, for example, X-ray images of the casing of the TMS coil device.
It is known that, in NBS, navigation accuracy and the accuracy of the determination of the E-field induced in the brain are greatly affected by any inaccuracies in the model of the coil windings within the casing of the TMS coil device. The manufacturer of a TMS coil device provides information on the location and orientation of the coil windings within the casing of a TMS coil device, which the manufacturer typically obtains by X-raying the casing of the TMS coil device, for use in NBS. The location and orientation information provided by the manufacturer corresponds to the location and orientation in the casing at which the coil windings are expected to be positioned during manufacture of the casing.
In presently available TMS coil devices, such as, for example, those sold by MAGSTIM and MEDTRONIC, however, the coil windings are not necessarily at the expected location within or orientation in relation to the casing, as indicated by the supplier of the devices, and in some circumstances, for example, are up to about 10 mm away from the expected location. The difference between the actual and expected locations of the coil windings is attributable to manufacturing tolerances relating to the placement of the coil windings within the casing. Also, the difference exists because, in the prior art, the coil windings ordinarily are positioned relatively freely within the casing of a TMS coil device. Thus, the coil winding location and orientation information provided for current TMS coil devices, which would then be relied upon for NBS, does not correspond with the actual location and orientation of the coil windings within the casing. It has been determined that, where the difference between the expected and actual locations of the coil windings in the casing is up to about 10 mm, inaccuracies in the computation of the location and strength of the E-field can be up to about 10 mm and greater than tenths of a percent, respectively.
In addition, during operation of a TMS coil device, resistive losses in the copper wire from which the coil windings are formed generate a substantial amount of heat energy relatively quickly, especially when the TMS coil device is operated to supply many sequential pulses to the brain. The heat energy generated in the coil windings raises the temperature of the coil windings and also the surrounding casing. Consequently, the temperature of the surface of the casing adjacent or in contact with the head of a subject increases as the heat energy is transferred to the casing. International standards, e.g., IEC-60601-1, require that the temperature of the outer surface of the casing of a TMS coil device should not exceed 41° C., such that the outer surface of the casing remains at a temperature considered to be safe for contact with a subject, such as a human or animal.
Thus, the temperature of the casing outer surface is a limit on the number of times that the TMS coil device can be sequentially pulsed. The current limitations on the number of pulses that can be sequentially applied by a TMS coil device restricts the potential applications of the TMS coil device, and also lengthens the time that a patient undergoing TMS must endure such procedure, which is undesirable.
In prior art TMS coil devices, the convection techniques of having fluid re-circulate about, or at least partially surround, the coil windings have been used to increase the rate of transfer of heat energy away from the coil windings. In addition, the fluid operates to decrease the rate of transfer of heat energy from the coil windings to the casing, and thus, decrease the rate at which the temperature of the casing outer surface increases. Such uses of fluid to increase the heat capacity of the TMS coil device, however, complicate the design and construction of the TMS coil device, which must be made waterproof, and also substantially increase the weight of the casing, which makes the TMS coil device more bulky and difficult to maneuver.
Further, in some prior art TMS coil devices, a reduction in the rate of heat energy transfer from the coil windings to the portion of the casing outer surface that would face or contact the subject's head is achieved because the coil windings are positioned a greater distance away from the casing portion than in other TMS coil devices. The distance of the coil windings from the subject's head, however, impacts the maximum E-field that can be induced in the brain. Therefore, it is disadvantageous for the coil windings to be positioned so far away from the casing outer surface portion that the maximum E-field that can be induced in the brain is undesirably diminished.
Therefore, there exists a need for a TMS coil device that can be manufactured with ease, repeatedly and inexpensively to provide that the coil windings are of a predetermined size and shape and are positioned, with great accuracy, at a predetermined location within and orientation in relation to a casing of the TMS coil device having a predetermined size and shape and, furthermore, to provide that the rate of transfer of heat energy from the coil windings to the casing outer surface is minimized without adversely impacting the maximum E-field that can be induced in the subject's brain.